Some examples of the conventional Y/C separation circuits will be described hereinbelow, in reference to FIGS. 1, 2, 3 and 4. Through the drawings, the same reference numerals or letters will be used to designate like or equivalent elements for simplicity of explanation.
FIG. 1 shows a typical example of a conventional Y/C separation circuit employing a comb filter configuration. In FIG. 1, a composite NTSC system video signal is applied to an adder 20, a subtractor 22 and a 1H delayer 24 through an input terminal 26. The composite video signal includes a luminance signal Y component and a chrominance signal C component. These components Y and C are modulated by a color sub-carrier frequency.
The 1H delayer 24 delays the composite video signal for a 1H period (H represents one horizontal scanning period). The 1H delay signal of the composite video signal obtained by the 1H delayer 24 is applied to both the adder 20 and the subtractor 22. The adder 20 adds the direct signal and the 1H delay signal of the composite video signal from the input terminal 26 and the 1H delayer 24, respectively, with each other. The subtractor 22 carries out a subtraction between the direct signal and the 1H delay signal of the composite video signal. Here, the frequency of the color sub-carrier frequency is an odd number times a half of the horizontal frequency. The phase of the color sub-carrier is inversed for every 1H period. The phase of the chrominance signal C is also inversed for every 1H period. Thus, the chrominance signal C components offset each other when the direct signal and the 1H delay signal of the composite video signal are added with each other in the adder 20. Thus, only the luminance signal Y component is left in the composite video signal. The luminance signal Y component thus left is output through a Y output terminal 28.
On the other hand, the luminance signal Y components offset each other when the direct signal and the 1H delay signal of the composite video signal are subtracted from each other in the subtractor 22. Thus, only the chrominance signal C component is left in the composite video signal. The chrominance signal C component thus left is output through a C output terminal 30.
In the typical example as mentioned above, the Y/C separation is carried out using the characteristics of the composite video signal of the NTSC system such that the luminance signal Y component has a strong correlation in the vertical direction and the phase of the chrominance signal C inverses for every 1H period.
For example, in case of vertical stripes which completely correlate with the luminance signal Y in the vertical direction, the Y/C separation can certainly be made by the circuit described above. However, where the composite video signal suddenly changes in the vertical direction, the Y/C separation using the comb filter malfunctions. That is, when the chrominance signal C changes suddenly, this signal change affects causes a dot disturbance in the luminance signal Y. When the luminance signal Y changes suddenly, this signal change also causes a cross-color distortion in the chrominance signal C.
In the conventional Y/C separation circuit, it is possible to determine whether the composite video signals on two adjacent horizontal scanning lines have a correlation with each other. However, it is impossible to effect the Y/C separation in the case where the composite video signals on two adjacent horizontal scanning lines have less correlation with each other in the vertical direction. So, further attempts to solve the problem have been made, employing two 1H delayers in such a comb filter.
FIG. 2 shows a second example of the conventional Y/C separation circuit which is disclosed in the Japanese Laid-open Patent Application P58-10913.
In FIG. 2, a composite video signal is applied to a minimum value detection circuit (referred to as MIN detector hereafter) 32, a maximum value detection circuit (referred to as MAX detector hereafter) 34 and a first 1H delayer 24a through an input terminal 26. The first 1H delayer 24a delays the composite video signal by 1H period. The output of the first 1H delayer 24a, i.e., the 1H delay signal of the composite video signal is applied to a second 1H delayer 24b, the MIN detector 32, the MAX detector 34, a first subtractor 36 and a second subtractor 38. The second 1H delayer 24b further delays the 1H delay signal of the composite video signal. The output of the second 1H delayer 24b, i.e., the 2H delay signal of the composite video signal is applied to the MIN detector 32 and the MAX detector 34.
The MIN detector 32 detects a minimum value of its three inputs, i.e., the direct signal, the 1H delay signal and the 2H delay signal of the composite video signals, which are supplied from the input terminal 26, the 1st 1H delayer 24a and the second 1H delayer 24b. The minimum value output (referred to as MIN signal hereafter) of the MIN detector 32 is applied to the first subtractor 36.
The MAX detector 34 detects a maximum value of its three inputs, i.e., the direct signal, the 1H delay signal and the 2H delay signal of the composite video signals, which are also supplied from the input terminal 26, the 1st 1H delayer 24a and the second 1H delayer 24b. The maximum value output (referred to as MAX signal hereafter) of the MAX detector 34 is applied to the second subtractor 38.
The first subtractor 36 carries out a subtraction between the MIN signal and the 1H delay signal of the composite video signal from the MIN detector 32 and the first 1H delayer 24a. The second subtractor 38 carries out a subtraction between the MAX signal and the 1H delay signal of the composite video signal from the MAX detector 34 and the first 1H delayer 24a. Difference signals output from the first and second subtractors 36 and 38 are applied to an adder 40. Thus, the adder 40 adds the difference signals with each other. A sum signal output from the adder 40 consists of a vertical non-correlative component, i.e., the chrominance signal C component of the composite video signal. This chrominance signal C component is supplied to a C output terminal 30 through an attenuator 42. The attenuator 42 attenuates the level of the chrominance signal C component output from the adder 40 to a predetermined level.
The chrominance signal C component and the 1H delay signal of the composite video signal from the attenuator 42 and the first 1H delayer 24a are applied to a third subtractor 44. Thus, the third subtractor 44 carries out a subtraction between the chrominance signal C component and the 1H delay signal of the composite video signal. The chrominance signal C component of the 1H delay signal of the composite video signal is offset by the chrominance signal C component from the attenuator 42 in the third subtractor 44. Thus, only the luminance signal Y component is left in the composite video signal. The luminance signal Y component thus left is output through a Y output terminal 28.
In this second conventional Y/C separation circuit, as shown in FIG. 2, the ability of detecting the correlation between two adjacent horizontal line signals has been improved by using two 1H delayers, i.e., the first and second 1H delayers 24a and 24b. As a result, the luminance signal Y component and the chrominance signal C component are certainly separated for non-correlative components in the vertical direction, reducing the dot disturbance and the cross-color distortion.
FIG. 3 shows a third example of the conventional Y/C separation circuit which is disclosed in the Japanese Laid-open Patent Application P63-59594.
In FIG. 3, a composite video signal is applied to a first comb filter consisting of a first 1H delayer 24a and a first subtractor 36, through an input terminal 26. The first 1H delayer 24a delays the composite video signal by 1H period. The output of the first 1H delayer 24a, i.e., the 1H delay signal of the composite video signal is applied to the first subtractor 36. The first subtractor 36 carries out a subtraction between the direct signal and the 1H delay signal of the composite video signal. A first difference signal output from the first subtractor 36 is applied to a correlation detector 46.
Further the 1H delay signal of the composite video signal from the first 1H delayer 24a is applied to a second comb filter consisting of a second 1H delayer 24b and a second subtractor 38. In the second comb filter, the 1H delay signal of the composite video signal is applied to both the second 1H delayer 24b and the second subtractor 38. The second 1H delayer 24b further delays the 1H delay signal of the composite video signal from the first 1H delayer 24a by 1H period. The output of the second 1H delayer 24b, i.e., the 2H delay signal of the composite video signal is also applied to the second subtractor 38. The second subtractor 38 carries out a subtraction between the 1H delay signal and the 2H delay signal of the composite video signal. A second difference signal output from the second subtractor 38 is applied to the correlation detector 46 through an inverter 48.
The correlation detector 46 detects a signal representing a correlation between the first and second difference signals. If these two difference signals have the same polarity, the correlation detector 46 outputs a correlation signal with less amplitude in either one of the two inputs. As seen from the above description, this situation corresponds to the case of the chrominance signal C component. If these two difference signals are opposite in phase, the correlation detector 46 fails to output the correlation signal. In other words, the level of the correlation signal becomes zero. This situation corresponds to the case of the luminance signal Y component. Thus, the correlation detector 46 selectively derives the chrominance signal C component. The chrominance signal C component is output through a C output terminal 30.
Further the chrominance signal C component derived from the correlation detector 46 is applied to a third subtractor 44. The 1H delay signal of the composite video signal from the first 1H delayer 24a is also applied to the third subtractor 44. Thus, the third subtractor 44 carries out a subtraction between the chrominance signal C component and the 1H delay signal of the composite video signal. The chrominance signal C component of the 1H delay signal of the composite video signal is offset by the chrominance signal C component from the correlation detector 46 in the third subtractor 44. Thus, only the luminance signal Y component is left in the composite video signal. The luminance signal Y component thus left is output through a Y output terminal 28.
FIG. 4 shows a fourth example of conventional Y/C separation circuit which is disclosed in the Japanese Laid-Open Patent Application P63-46088.
In FIG. 4, a composite video signal is applied to a first comb filter consisting of a first 1H delayer 24a and a first subtractor 36, through an input terminal 26. The first 1H delayer 24a delays the composite video signal by 1H period. The output of the first 1H delayer 24a, i.e., the 1H delay signal of the composite video signal is applied to the first subtractor 36. The first subtractor 36 carries out a subtraction between the direct signal and the 1H delay signal of the composite video signal. A first difference signal output from the first subtractor 36 is applied to a first controllable resistor 50 which will be described later. The first difference signal represents a first signal of the chrominance signal C component.
Further the direct signal and the 1H delay signal of the composite video signal are applied to a first adder 52. The first adder 52 adds the direct signal and the delay signal of the composite video signal with each other. A first sum signal output from the first adder 52 represents a correlation between the direct signal and the 1H delay signal of the composite video signal. The level of the first sum signal, i.e., a first correlation signal assigned to the composite video signal, lowers in proportion to an amount of the correlation. For example, an "n"th horizontal line signal and its 1H prior signal, i.e., an "n-1"th horizontal line signal.
The first correlation signal is applied to a first rectifier 54 through a first band pass filter (referred to as BPF hereafter) 56. The first rectifier 54 rectifies the first correlation signal so that a first DC correlation signal is obtained. The first DC correlation signal is applied to a control terminal of the first controllable resistor 50.
The 1H delay signal of the composite video signal from the first 1H delayer 24a is applied to a second comb filter consisting of a second 1H delayer 24b and a second subtractor 38. The second 1H delayer 24b delays the 1H delay signal of the composite video signal by 1H period. The output of the second 1H delayer 24b, i.e., the 2H delay signal of the composite video signal is applied to the second subtractor 38. The second subtractor 38 carries out a subtraction between the 1H delay signal and the 2H delay signal of the composite video signal. A second difference signal output from the second subtractor 38 is applied to a second controllable resistor 58 which will be described later. The second difference signal represents a second signal of the chrominance signal C component.
Further the 1H delay signal and the 2H delay signal of the composite video signal are applied to a second adder 60. The second adder 60 adds the 1H delay signal and the 2H delay signal of the composite video signal with each other. A second sum signal output from the second adder 60 represents a correlation between the 1H delay signal and the 2H delay signal of the composite video signal. The level of the second sum signal, i.e., a second correlation signal assigned to the composite video signal lowers in proportion to an amount of the correlation. For example, an "n-1"th horizontal line signal and its 1H prior signal, i.e., an "n-2"th horizontal line signal.
The second correlation signal is applied to a second rectifier 62 through a second BPF 64. The second rectifier 62 rectifies the second correlation signal so that a second DC correlation signal is obtained. The second DC correlation signal is applied to a control terminal of the second controllable resistor 58.
The first and second controllable resistors 50 and 58 attenuate the first and second signals of the chrominance signal C component from the first and second subtractors 36 and 38, in proportion to the first and second DC correlation signals, respectively. Outputs of the first and second controllable resistors 50 and 58 are coupled in common to a buffer 66. Thus, the first and second signals of the chrominance signal C component are integrated by the buffer 66. The output of the buffer 66 is passed through a third BPF 68 as an integrated signal of the chrominance signal C component. The integrated signal of the chrominance signal C component is then output through a C output terminal 30. Further the integrated signal of the chrominance signal C component is applied to a third subtractor 44 which will be described later.
Now assuming the "n-1"th and the "n-2"th horizontal line signals have a high correlation with each other than the correlation between the "n"th and the "n-1"th horizontal line signals, the first signal of the chrominance signal C component from the first subtractor 36 dominates in the input of the buffer 66 over the second signal of the chrominance signal C component from the second subtractor 38. Whereas the second signal of the chrominance signal C component dominates in the input of the buffer 66 over the first signal of the chrominance signal C component, when the "n"th and the "n-1"th horizontal line signals have a high correlation with each other than the correlation between the "n-1"th and the "n-2"th horizontal line signals. Thus, the correlations of the three adjacent horizontal line signals reflect well on the integrated signal of the chrominance signal C component.
Further the integrated signal of the chrominance signal C component is applied to the third subtractor 44. The 1H delay signal of the composite video signal from the first 1H delayer 24a is also applied to the third subtractor 44 through a timing adjuster 70. Thus, the third subtractor 44 carries out a subtraction between the integrated signal of the chrominance signal C component from the third BPF 68 and the 1H delay signal of the composite video signal. The chrominance signal C component of the 1H delay signal of the composite video signal is offset by the integrated signal of the chrominance signal C component in the third subtractor 44. Thus, only the luminance signal Y component is left in the composite video signal. The luminance signal Y component thus left is output through a Y output terminal 28.
As described above, the first conventional Y/C separation circuit, which employs a single 1H delayer (see FIG. 1), is capable of determining whether the composite video signals on two adjacent horizontal scanning lines have a correlation with each other. Further, the second, third and fourth conventional Y/C separation circuits which employ two 1H delayers (see FIGS. 2, 3, 4) are capable of certainly reducing cross-color distortion and dot disturbance and especially, as far as the dot disturbance is concerned, they have a very satisfactory characteristic.
However, these conventional Y/C separation circuits have problems, such as high cost and deterioration of the S/N ratio and horizontal resolution. The deterioration of the S/N ratio and horizontal resolution are caused by the 1H delayers. That is, in the second, third and fourth examples as shown in FIGS. 2, 3 and 4, the luminance signal Y component is inevitably derived from the composite video signal passing through the first 1H delayer 24a.
Here the luminance signal Y component is requested to have a sufficiently wide frequency band and a high S/N ratio. Therefore, the 1H delayers in the conventional Y/C separation circuits are required to use charge coupled devices (referred as to CCD devices, hereafter) for satisfying the wide frequency band and the high S/N ratio. However, the S/N ratio of the CCD devices is not sufficient for satisfying the quality of display images desired in recent high-definition video systems.
The clock frequency for operating a wide band CCD device is as high as, e.g., 14 MHz. Therefore, the upper frequency of the signal capable of passing through the CCD device is limited to 5 to 6 MHz. The same may be true when a digital memory is used. The upper frequencies, i.e., 5 to 6 MHz, are equivalent to the horizontal resolutions of 400 to 480 lines. In recent television receivers with a wide screen, the horizontal resolution of the input video signal is heightened to 600 lines or above. However, such a high horizontal resolution cannot be achieved by the conventional Y/C separation circuits described above.
Furthermore, such a wide band CCD device is very expensive and therefore, there was such a problem in that the cost would become extremely high in comparison to a case where a comb filter which allows the chrominance signal C component only to pass through the 1H delayer was adopted.
In the conventional Y/C separation circuits of FIGS. 2, 3 and 4, three adjacent horizontal line signals, i.e., the "n"th, "n-1"th and "n-2"th horizontal line signals from the input terminal 26, the first and second 1H delayers 24a and 24b are operated to derive the chrominance signal C component. However, malfunctions may possibly occur due to the low band luminance signal Y component contained in the output of the 1H delayers. So, in order to perform the operation in the frequency band of the chrominance signal C (3 to 4 MHz, in case of NTSC signal) only, a BPF (not shown in FIGS. 2, 3 and 4) is provided for limiting the frequency band of the composite video signal. Here, as described above, the luminance signal Y component inevitably passes through such 1H delayers. Thus, it is not possible to locate a common BPF in a position prior the 1H delayers. Thus, three BPFs must be used for the three adjacent line signals. This necessity of three BPFs also causes the cost to increase.